MULTILAYER STRUCTURE WITH HIGH ORDERED FePt LAYER

ABSTRACT

A multilayer structure and method for making the same. In accordance with some embodiments, a multilayer structure has a first layer of Fe, a layer of A1 phase FePt on the first layer of Fe, and a second layer o Fe on the layer of FePt. The multilayer structure is annealed to convert the A1 phase FePt to L1o phase FePt.

RELATED APPLICATION

This application is a divisional of copending U.S. patent application Ser. No. 12/369,844 tiled on Feb. 12. 2009.

BACKGROUND

Materials with increased magnetic anisotropies are desirable for various applications such as, for example, applications in the data storage industry where there is a continuous need to increase storage densities. Data storage media that can hold densities approaching 1 Tbit/in² will require materials with magnetic anisotropies greater than conventional media materials. There are known hulk permanent magnetic materials having crystalline phases with magnetocrystalline anisotropy that theoretically can hold densities greater than 1 Tbit/in². For bulk permanent magnetic materials, special heat treatments are typically used to control the phase formation and microstructure to optimize the material properties. In order to incorporate these materials into a data storage media, the correct crystalline phase must be obtained within a microstructure of tine, nanocrystalline, exchange decoupled or partially exchange decoupled grains while maintaining thermal stability.

L1₀ phase FePt binary alloys have magnetocrystalline anisotropy as high as 7×10⁷ erg/cc, which is well suitable for future magnetic recording media to achieve density over 1 Tb/in². However, FePt is typically deposited as the face centered cubic (fcc) phase (i.e., the A1 phase) and subsequent annealing is needed to transform (i.e., chemically order) the material into the high anisotropy L1₀ phase.

This high temperature processing is likely to enhance grain growth. which is opposite to the small grain size requirement for high density recording. On the other hand, fully ordered FePt media generally have a coercivity over 4 Tesla, which is beyond current writer technology capabilities. It would he desirable to produce FePt media with a small grain size d with magnetic characteristics that c compatible with current writer technology.

SUMMARY

Various embodiments are generally directed to a multilayer structure and method for making the same. In accordance with some embodiments, a multilayer structure comprises a first layer of Fe, a layer or A1 phase FePt on the first layer of Fe. and a second layer of Fe on the layer of FePt. The multilayer structure is annealed to convert the A1 phase FePt to L1o phase FePt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a material stack used in a first aspect of the invention.

FIG. 2 is a graph of a typical hysteresis loop for materials that can he used in the material stack of FIG. 1.

FIG. 3 is a side elevation view of another material c used in another aspect of the invention.

FIG. 4 is a graph of a typical hysteresis loop for materials that can be used in the material stack of FIG. 3.

FIG. 5 is a schematic representation of a granular media.

FIG. 6 is a schematic representation of a process for producing exchange coupled composite media.

FIG. 7 is a schematic representation of a process for producing graded anisotropy media.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, the invention provides a method of fabricating a data storage media that uses a multilayer structure including seed and cap layers of either Fe or Pt on opposite sides of an A1 phase FePt layer, and anneals the multilayer structure to convert the A1 phase FePt to L1_(o), phase FePt at a relatively low anneal temperature.

FIG. 1 is a side elevation view of a material stack 10 used in a first aspect of the invention. A multilayer structure 12 is formed on a substrate 14. The multilayer structure includes a first layer 16 (e.g., a seed layer) of Pt, a first layer 18 of A1 phase FePt on the first layer of Pt, and a second layer 20 (e.g., a cap layer) of Pt on the layer of FePt. The substrate can be, for example, glass, aluminum or its alloys. etc.

The layers can be formed by physical vapor deposition methods, such as magnetron sputtering, pulsed laser deposition, or ion beam deposition.

In one example, layer 16 can have a thickness ranging from about 0.5 nm to about 2 nm. Layer 18 can have a thickness ranging from about 2 nm to about 10 nm. Layer 20 can have a thickness ranging from about 0 nm to about 5 nm.

The multilayer structure is annealed to convert the A1 phase FePt to L1_(o) phase FePt. In one example, the annealing step can be performed at about 300° C. for 4 hours. In other examples, the minimum annealing temperature can be in a range from about 200° C. to about 500° C. The annealing temperature needed to convert the A1 phase FePt to L1_(o) phase FePt will depend on the layer thicknesses.

FIG. 2 is a graph of magnetization versus applied magnetic field (i.e., a hysteresis loop) for a multilayer structure as shown in FIG. 1 in which the FePt layer comprises Fe₆₂Pt₃₈. During annealing, there will be some diffusion between the layers, which will depend on annealing temperature used.

FIG. 2 shows a coercivity of about 9130 Oe for the multilayer structure. The hysteresis loop was measured by monitoring the magnetic moment change while the applied field is sweeping from −18000 Oe to +18000 Oe and then swept backwards using a vibrating sample magnetometer (VSM) at room temperature. The magnetic field was applied with its direction parallel to the film plane, which is called an in-plane measurement, and the coercivity is then called the in-plane coercivity.

FIG. 3 is a side elevation view of a material stack 30 used in another aspect of the invention. A multilayer structure 32 is formed on a substrate 34. The multilayer structure includes a first layer 36 (e.g., a seed layer) of Fe, a first layer 38 of A1 phase FePt on the first layer of Fe, and a second layer 40 (e.g., a cap layer) of Fe on the layer of FePt. The multilayer structure is annealed to convert the A1 phase FePt to L1_(o) phase FePt. The substrate can be, for example, glass, aluminum or its alloys, etc.

In one example, layer 36 can have a thickness ranging from about 0.5 nm to about 2 nm. Layer 38 can have a thickness ranging from about 2 nm to about 10 nm. Layer 40 can have a thickness ranging from about 0 nm to about 5 nm.

The multilayer structure is annealed to convert the A1 phase FePt to L1_(o) phase FePt. In one example, the annealing step can be performed at about 300° C. for 4 hours. In other examples, the minimum annealing temperature can he in a range from about 200° C. to about 500° C. The annealing temperature needed to convert the A1 phase FePt to L1_(o) phase FePt will depend on the layer thicknesses.

FIG. 4 is a graph of magnetization versus applied magnetic field for a multilayer structure of FIG. 3 in which the FePt layer comprises Fe₃₆Pt₆₄. FIG. 4 shows a coercivity of about 1800 Oe for the multilayer structure.

In the examples of FIGS. 1 and 3, atom diffusion at the two interfaces, between the seed layer and the FePt layer, and between the cap layer and FePt layer. facilitates the chemical ordering process for the whole stack.

The platinum seed and cap combination of FIG. 1 is suitable for an iron rich FePt alloy middle layer, as diffusion will drive the whole film towards the stoichiometric composition of 50 to 50. In one example using platinum as the seed and cap layer, an in-plane coercivity value of 9130 Oe has been achieved after annealing at a temperature as low as 300° C. Similarly, the iron seed and cap combination of FIG. 3 is effective for ordering a platinum rich FePt alloy middle layer.

Media grain size is reduced as the annealing temperature decreases. Thus the use of a low annealing temperature would limit the media grain site. A successful low temperature phase transformation method ensures small grain size for high recording areal density.

Since the inter-diffusion occurs in a direction perpendicular to the plane of the films, grain isolation materials such as SiO₂, carbon, boron, or other oxide or nitride material can be applied to keep the FePt grain size within several nm. These grain isolation materials can he applied either by embedding them into the target material. or through co-sputtering from a separate target in the same chamber.

FIG. 5 is a schematic drawing of a media 42 having an FePt layer 44 between Pt seed and cap layers 46, 48. Grain isolation material 50 is shown to be positioned between grains 52 in the stack.

In another aspect, the invention provides data storage media with enhanced writability. One way to enhance writability is to form so-called Exchange-Coupled Composite (FCC) media. FCC media include one magnetically hard phase (in this case low temperature ordered L1₀ FePt) and one magnetically soft phase, which can be disordered FePt or other high magnetization materials such as FeNi, FeCo, etc. As used in this description. a magnetically hard material is a material that typically has a coercive force higher than 2000 Oe, and a magnetically soft material is a material that typically has a coercive force lower than 2000 Oe.

To make FCC media, L1₀ FePt is prepared at low temperature to keep the grain size small. as described above. Then the top Pt (or Fe) cap layer is removed and the soft phase material is deposited on the L1₀ FePt. An optional thin exchange coupling control layer of another material can be deposited before the soft phase material such that the exchange coupling control layer is positioned between the hard and soft layers to tune the interlayer exchange coupling strength. The exchange coupling control layer can be, for example, Pt, PtSi, Pd, or PdSi.

FIG. 6 is a schematic representation of a process for producing exchange coupled composite media. The process starts with a material stack 54 including a FePt layer 56 sandwiched in between two Pt layers 58, 60, which is annealed to convert the FePt to the L1₀ phase in later 62. Next, the cap layer is removed, for example, by plasma etching (e.g., sputtering) or ion beam milling (e.g., ion bombardment). Then a magnetically softer layer 64 is deposited onto the ordered FePt hard layer. Optionally, a thin layer 66 of Pt, Pd, or some non-magnetic material such as silicon containing material, can be deposited on the ordered FePt hard layer prior to the deposition of c magnetically softer layer to control the exchange coupling between the soft or hard layer. The bottom Pt partially diffuses into FePt, any leftover of Pt would be minimal so it should not affect media property.

FIG. 6 shows the use of an iron rich middle layer that is converted to a stoichiometric composition of 50 to 50. Such conversion can be implemented by annealing or by elevated temperature deposition, wherein the media stack is deposited at temperatures higher than room temperature.

In another aspect, the invention provides graded anisotropy (GA) media, which has been proposed theoretically to address the writability issue. Inter-diffusion of iron and platinum atoms is used to produce a composition gradient as well as a chemical ordering gradient, which in turn provides an anisotropy gradient.

FIG. 7 is a schematic representation of a process for producing graded anisotropy media. The process starts with a material ck 70 including an iron rich FePt layer 72 sandwiched in between a Pt seed layer 74 and a Fe cap layer 70. The stack is annealed to produce a graded layer 78 having a substantially stoichiometric composition of 50 to 50 near the bottom and an iron rich composition near the top. Such conversion can also be implemented by elevated temperature deposition, wherein the media stack is deposited at temperatures higher than room temperature.

It is well known that a concentration gradient can help the diffusion of atoms. In the annealing process, thermal energy helps to create vacancies and enhance the mobility of atoms to allow for atomic reorganization. The gradient can be tuned by adjusting seed and cap thickness, as well as by adjusting the annealing temperature or time.

FePt media anisotropy strongly depends on the composition, and maximizes around Fe₅₀₋₅₅Pt₅₀₋₄₅. The anisotropy decreases when the material composition is off stoichiometry. For the purposes of this description, perfect L1₀ FePt stoichiometry refers to Fe₅₀₋₅₅Pt₅₀₋₄₅.

With the process of FIG. 7, it is possible to establish gradient anisotropy media with a composition gradient. This can be, for example, achieved by using Pt diffusion from the bottom and Fe diffusion from the top layer, and results in close to equal stoichemistry at the bottom (high anisotropy K) and an Fe rich composition on the top (low anisotropy K). A similar approach can be implemented by sandwiching iron deficient FePt film between a Fe seed layer and a Pt cap layer, with Fe diffusion from the bottom and Pt diffusion from the top.

While the invention has been described in terms of several examples, it will be apparent to those skilled in the art that various changes can he made to the disclosed examples, without departing from the scope of the invention as set forth in the following claims. The implementations described above and other implementations are within the scope of the following claims. 

1. A method comprising: constructing a multi layer structure comprising a first layer of Fe, a layer of A1 phase FePt on the first layer of Fe, and a second layer of Fe on the layer of FePt; and annealing the multilayer structure convert the A1 phase FePt to L1o phase FePt.
 2. The method of claim 1, in which the annealing is performed at about 300° C.
 3. The method of claim 1, in which the A1 phase FePt is Pt rich and comprises fe50−xPt50+x, where x is larger than 5 but less than
 30. 4. The method of claim 1, further comprising removing the second layer Fe, and depositing a layer of magnetically soft material on the L1o phase FePt layer.
 5. The method of claim 1, further comprising removing the second layer of Fe, depositing an exchange control layer on the L1o phase FePt, and depositing a layer of magnetically soft material on the exchange control layer.
 6. The method of claim 5, in which the exchange control layer comprises at least one of Pt, Pd, or a non-magnetic material.
 7. The method of claim 1, in which the multilayer structure further comprises a grain isolation material.
 8. The method of claim 1, in which the multi layer structure is constructed by physical vapor deposition.
 9. The method of claim 1, in which the annealing is performed using a minimum annealing temperature in a range of from about 200° C. to about 500° C.
 10. The method of claim 1, in which the multilayer structure is characterized as a recording layer of a magnetic recording medium.
 11. A multilayer structure comprising a First layer of Fe, an intermediary layer of FePt characterized as L1o phase FePt, and a second layer of Fe on the intermediary layer.
 12. The multilayer structure of claim 11, in which the layer of FePt is transitioned from an A1 phase to the L1o phase by annealing the structure at a minimum annealing temperature of from about 200° C. to about 500° C.
 14. The multilayer structure of claim 11, further comprising a magnetically soft top layer on the second Fe layer.
 15. The multilayer structure of claim 14, further comprising an exchange control layer between the L1o phase FePt layer and the magnetically soft top layer.
 16. The multilayer structure of claim 15, in which the exchange control layer comprises at least one of Pt, Pd, or a non-magnetic material.
 17. The multilayer structure of claim 11, characterized as exchange coupled composite (ECC) media.
 18. The multilayer structure of claim 11, in which the multilayer structure is characterized as a recording layer of a magnetic recording medium.
 19. A multilayer structure comprising: a substrate; and means for storing data disposed over the substrate.
 20. The multilayer structure of claim 19, in which the means for storing data comprises respectively ordered layers of Pt—FePt—Pt; Fe—FePt—Fe; Pt—FaPt—Fe; or Fe—FaPt—Pt. 